Maintenance of an inert (or protective) gas in the vapor space over a flammable liquid held in a storage tank prevents atmospheric air from entering the tank. This minimizes liquid evaporation and environmental emissions, reduces fire and explosion risks, and also prevents a buildup of vacuum (subatmospheric pressures) in tank vapor space that is in excess of the tank's design capacity. To achieve such maintenance, tank blanketing valve assemblies that incorporate vapor space gas pressure sensing and regulating means have come into widespread use. Such a valve assembly is functionally associated with a top portion of a storage tank along with an independently functioning emergency pressure vent and an independently functioning conservation vent. The latter vent may be in association with a flame arrestor.
Tank blanketing valve assemblies often incorporate a main valve and a cooperating pilot valve operating in tandem. In such an assembly, a sensed pressure change in the tank gas causes a responsive movement in the pilot valve that opens the main valve and allows inert gas to flow into the tank.
Prior art tank blanketing valves suffer from various problems. One problem is that in an assembled tank blanketing valve, it was not previously possible to conveniently adjust the internal operation or position of pilot valve functional components. The pilot valve and the main valve components had to be precisely machined to achieve a predetermined positional relationship between components. Upon assembly, even slight variations in machining tolerances, operating conditions or even environmental conditions could change the desired operating characteristics of the assembled tank blanketing valve. There was no known way to adjust the positioning of the internal functional components of the pilot valve relative to one another from an external location after valve assembly.
Another problem is that an O-ring seal associated with the pilot valve assembly located entirely within the assembled tank blanketing valve but adjacent to the main valve dome chamber had a tendency to unseat during main valve operation. The unseating occurs as a result of pressure differentials that act upon the seal. The unseating results in excessive inert gas leakage. Reseating of this O-ring can only be accomplished by dismantling the entire tank blanketing valve.
The present invention provides a tank blanketing valve structure which overcomes the foregoing problems.